In high-volume production of baked goods, for example cookies, crackers, bread, rolls etc., baking of the goods is conventionally effected using a band oven. Such a band oven comprises an insulated housing enclosing a baking chamber of considerable length; chambers in commercial ovens are typically around 300 feet (approximately 90 meters) long. A belt conveyor formed of a heat-resistant material, typically steel mesh, extends longitudinally through the baking chamber and extends beyond both ends of the baking chamber. Goods to be baked are placed on the inlet end of the belt conveyor, which moves continuously and thus carries a continuous stream of goods through the baking chamber. The speed of the conveyor is regulated so that the goods being baked remain within the baking chamber for the proper baking time, and the baked goods are continuously discharged from the outlet end of the conveyor.
Heating of the baking chamber is conventionally effected by means of elongate gas burners which extend transversely across the conveyor at spaced intervals, each of these gas burners being provided with a row of apertures through which gas issues to form either a large number of individual flames lying close to one another, or a single continuous ribbon-like flame extending the full length of the row of apertures. Either form of flame will hereinafter be referred to as a "ribbon flame". To render supervision and maintenance of the band oven as simple as possible, one side of the housing is provided with a series of removable plates, one adjacent the end of each gas burner, these removable plates each bearing a gas supply aperture through which gas is supplied to the burner and a transparent window through which an operator can observe the gas burner to ensure that proper combustion is taking place. The plate may also be provided with an inlet for power connections to an igniter placed adjacent the gas burner to ignite the gas issuing from the burner.
A major consideration in the operation of such band ovens is to ensure uniformity of heating along the band, since if non-uniform distribution of heat occurs, with the development of hot zones on the band adjacent each burner and cooler spots midway between adjacent burners, the baking of the goods may be deleteriously affected, and excessive gas consumption may occur. (References to non-uniformity of heating herein refer to such non-uniformity of heating over the relatively short distances between adjacent gas burners and are not intended to refer to deliberate creation of zones of differing temperature along the length of the band oven which are often deliberately introduced, for example to ensure that the goods entering the oven are rapidly raised to a proper baking temperature.) Non-uniformity of heating can be reduced by using a large number of burners spaced apart by relatively small distances. For example, in prior art band ovens burners may be spaced at intervals of 12 to 18 inches (305 to 457 mm.) along the length of the band, the burners being disposed in two rows above and below the band.
Unfortunately, when such a large number of burners are employed the rate at which gas needs to be burned at each burner to maintain the requisite baking temperature within the insulated baking chamber is low, so that only a short flame is produced at each burner, and the burner thus provides a concentrated heat source, thereby tending to produce non-uniformity of heating within the baking chamber. Thus, improvement in the uniformity of heating within band ovens is desirable.
U.S. Pat. No. 596,578 describes an acetylene burner in which gas is forced under pressure through a duct into a slot where it comes into contact with air contained within the slot and contacts a flat vertical wall of the slot opposite the side to which the gas enters so that the column of gas is flattened. This flattened column of gas, following upwardly along the vertical face of the wall of the slot, passes out of the slot, where it is ignited and a flat, uniform and smokeless blaze is produced.
U.S. Pat. No. 1,213,675 to Maynard describes an oil burner having a slot-like aperture through which the oil is discharged, and a steam conduit through which steam passes into a large number of apertures, which emerge from the burner adjacent the slot-like oil aperture, so that the steam is projected from the burner in a substantially fan-shaped form contacting and mixing with oil projected through the oil conduit to atomize the oil.
U.S. Pat. No. 1,400,024 to Caracristi describes a gas burner in which control of the gas/air ratio is effected by providing an air supply conduit, which is separate from the gas supply conduit, and through which air is forced under pressure. The combustible gas emerges from the burner through a set of slot-like gas apertures arranged parallel to one another, while the air emerges through a set of slot-like air apertures disposed between adjacent pairs of the gas apertures, so that parallel alternating flat streams of gas and air emerge from the burner. It is stated that the alternate layers of gas and air emerging from the slotted outlets form a stratified stream at the exit, the gas and air then becoming thoroughly intermixed so that the regulation of the air both as to velocity, pressure and volume enables a proper quantity of air to be provided depending upon the quality of the gas, thus providing maximum combustion effeciency.
U.S. Pat. No. 1,702,625 to Anderson et al. describes a gas burner having the form of an elongate trapezoidal prism. This prism is hollow, having an internal trapezoidal chamber open at both top and bottom, flanked by two almost triangular prismatic gas chambers closed at their lower ends but having narrow, slot-like apertures at their upper ends. The jets of combustible gas emerging from the upper ends of these two gas chambers are angled inwardly towards one another so that they meet along a line lying above the central chamber of the gas burner. This arrangement allows air to be drawn up through the central chamber of the gas burner, thereby augmenting the air supply along the line at which the gas jets meet and, so the patent states, producing an intensely hot flame tip along this line.
U.S. Pat. No. 2,418,533 to Walker describes a gas torch designed so that it is capable of being operated very hot with no possibility of flashbacks and so that high velocity and highly oxidizing flame jets can be employed without danger of the flames blowing away from the tip or blowing out. This gas torch has a tip of elongate rectangular form having a slot of rectangular cross-section extending longitudinally therein. Combustible gas is supplied to two gas conduits each of which communicates with the base of the slot via a row of apertures, the two rows of apertures being on opposed sides of the slot. Oxygen is supplied through a third conduit from which extend a row of branch conduits, this row of branch conduits running parallel to the length of the slot. However, the oxygen branch conduits do not open into the slot, as do the gas conduits, but instead terminate in nozzles which extend upwardly along the central plane of symmetry of the slot and terminate flush with the outer surface of the slot.
U.S. Pat. No. 2,638,159 to Winkleman et al. describes a gas burner intended for producing very high temperatures for use in stripping coatings such as rust, scale, paint and other organic coatings from the surfaces of wood, metal and other bodies. In this burner, combustible gas emerges from a single row of apertures. Oxidizing gas is supplied via two rows of apertures, parallel to each other and to the row of apertures for the combustible gas, both rows of oxidizing gas apertures lying on the same side as the combustible gas apertures. The two sets of conduits which terminate in the two rows of oxidizing gas apertures are angled both with respect to each other and with respect to the set of conduits which terminate in the combustible gas apertures, so that the stream of combustible gas and the two streams of oxidizing gas intersect along a single line. This retards the velocity at which the oxidizing gas is supplied to the surface coating to be treated, and thus causes the oxidation action of the burner to be uniform over the width covered by the burner, rather than concentrated in narrow paths corresponding to separate jets of oxidizing gas.
U.S. Pat. No. 2,911,035 to Nieman et al. describes a gas burner for producing a soft, silent flame of extremely high temperature. In this burner, a single casing is provided with a large number of apertures, some being connected to the combustible gas supply and the remainder to the oxygen supply, the oxygen and combustible gas apertures being intimately mixed together so that mixing of oxygen and combustible gas takes place only after the gases have left the burner, thereby preventing flashback. However, the oxygen and combustible gas apertures are so close together that intimate mixing takes place almost at once and complete burning takes place with a very short distance from the plate.
U.S. Pat. No. 3,418,062 to Hovis et al. describes a substantially cylindrical burner intended for use in a soaking pit. In this burner, a central fluid fuel outlet is surrounded by two concentric rings of combustion air outlets. The fuel outlet and the inner ring of gas outlets direct fuel and air respectively parallel to the axis of the burner, while the outer ring of gas outlets projects jets of air diverging away from the axis of the burner.
U.S. Pat. No. 3,706,520 to Grimm et al. describes a complicated fuel gas burner for a vertical shaft furnace. This burner includes a single shaft containing four concentric conduits, the conduits containing fuel, air, fuel and air respectively reading from the innermost conduit. The three inner conduits terminate in separate plena each provided with a plurality of outlets arranged to direct the fuel or air substantially perpendicular to the axis of the shaft. The outermost conduit opens into a jacket which directs the air backwards in the opposite direction to the main fuel and air flow, so that the outermost air is directed backwards parallel to the axis of the shaft.
U.S. Pat. No. 3,850,571 to Zink et al. describes a high energy flame burner intended to produce a long, slender, rod-jet of flame of small diameter. The flame burner has a substantially cylindrical tube the tip of which is provided with a principal gas outlet orifice through which a combustible gas jet flows axially of the burner. In the side walls of the burner are cut a plurality of secondary gas outlets, which issue into a collar surrounding the burner. The forward end of this collar is provided with a forwardly-diverging frusto-conical flange, while the rear wall of the collar is provided with a plurality of apertures. The flow of combustible gas through the secondary gas apertures draws air through the apertures in the rear wall of the collar and the resultant mixture of gas and air is caused by the frusto-conical flange to issue from the forward end of the collar as an outwardly diverging ring of flame encasing the principal gas jet so as to assist in the ignition of the gas in the principal jet, and prevent it from being blown out due to the high velocity of the jet.
None of the above patents indicate any way in which the ribbon gas jet from a gas burner used in a band oven could be modified so as to improve the uniformity of heating within the band oven, and there is thus still a need for a solution to the problem of non-uniformity of heat distribution in band ovens. The present invention provides a way of improving the uniformity of heat distribution in a band oven.